1. Field of the Invention
The present invention relates to a frequency converter, more particularly to a frequency converter used for radio communications.
2. Description of the Related Art
A direct conversion receiving system suitable for miniaturization due to the reduced number of parts has been put to practical use with the penetration of radio communication terminal such as portable telephone. However, the direct conversion system uses a local oscillation signal (LO signal) of substantially the same frequency as the receiver signal (RF signal) frequency that is received by a radio antenna. For this reason, the direct conversion system includes a problem such as a self-mixture that the leakage LO signal enters in a receiver signal channel and becomes a jamming signal. As one method to solve this is suggested a method to use LO signal of approximately half frequency of the receiver signal. According to this method, approximately half frequency of RF signal frequency is used as LO signal frequency.
A direct conversion receiver comprises an even harmonic mixer using half frequency of RF signal frequency. This direct conversion receiver has property suitable for the direct conversion because this receiver does not indicate theoretically sensitivity to the LO signal even if the LO signal frequency enters in the receiver signal channel. However, the even harmonic mixer includes a problem that when the LO signal amplitude fluctuates by temperature change, the conversion gain has varied with this fluctuation, that is, the gain of the receiver does not indicate a target value.
Further, the gain greatly varies when the LO signal generator which is sensitive to the temperature change is used. In order to solve this problem, the conventional device has employed a method of inputting the LO signal to a limiting circuit to make the amplitude constant, and then inputting the LO signal to an even harmonic mixer. However, this method needs a filter for eliminating the harmonic component because many harmonic components of the LO signal occur in the limiting circuit. Generally it is difficult to integrate this filter on a chip.
Further, it obstructs an advantage of the direct conversion that the number of parts can be reduced to provide the filter between the LO signal generator and the even harmonic mixer. Since the conversion gain of the even harmonic mixer depends upon fluctuation of the LO signal amplitude, it is difficult to combine the even harmonic mixer with the cheap oscillation circuit by which the LO signal amplitude is fluctuated in easy.
It is an object of the present invention to provide a frequency converter that can utilize an advantage of an even harmonic mixer that the sensitivity deterioration due to self-mixture is small, without increase of parts.